This application is based on Application No. 2001-157284, filed in Japan on May 25, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
This invention relates to a fuel nature judging device for an internal combustion engine for judging the nature of a fuel supplied to an engine (internal combustion engine).
2. Description of the Related Art
Conventionally known fuel control devices include a fuel control device which detects the engine cooling water temperature (engine temperature) at the start of an engine (internal combustion engine) or immediately after the start of the engine to determine the fuel amount at the time of engine start in accordance with the engine temperature, and a fuel control device which determines the fuel amount increase value in accordance with the engine temperature at the start of an engine and which, immediately after the start of the engine, gradually reduces the fuel amount increase value with passage of time.
Apart from this, an ignition timing control device is conventionally known which advances ignition timing with respect to a reference value in accordance with the engine temperature detected immediately after the start of the engine.
Further, among conventionally known idle air amount control devices are an idle air amount control device which determines the air amount at the time of engine start in accordance with the engine temperature at the time of engine start or immediately after the engine start, and an idle air amount control device which determines the increase in air amount in accordance with the engine temperature at the time of engine start and which, immediately after the engine start, gradually reduces the air amount increase value with passage of time.
It is to be noted that in the conventional internal combustion engine control devices as mentioned above, no special consideration is given to the nature of the fuel with respect to fuel amount control, ignition timing control, and idle air amount control. In these conventional control devices, the fuel amount, ignition timing, and idle air amount at the time of engine start or immediately after the engine start are adjusted to a specific fuel nature.
On the other hand, recently, there is a demand for an internal combustion engine capable of always ensuring a stable combustion independently of the nature of the fuel, and there have been proposed various kinds of fuel nature judging devices for indirectly judging the fuel nature.
Specifically, the nature of the fuel constitutes an issue when, for example, a heavy gasoline is used. The heavy gasoline is one having a high distillation temperature and a rather low volatility. Its vapor pressure (RVP) is low, and its specific gravity and octane value are higher than those of a regular gasoline (ordinary gasoline).
However, in terms of standardization, there is no distinction between the heavy gasoline and the ordinary gasoline, and no distinction is made at the time of fuel supply as in the case of high-octane gasoline, regular gasoline, etc.
Thus, it can happen that a heavy gasoline is supplied to a vehicle equipped with an internal combustion engine control device for performing fuel amount control, ignition timing control, and idle air amount control in a manner suitable for the regular gasoline.
When a heavy gasoline is supplied to a vehicle equipped with an internal combustion engine control device adapted for regular gasoline, instability in engine rotating speed at the time of engine start and immediately after the engine start, hesitation on acceleration during running, etc. constitute a problem. Further, it can happen that the engine is incapable of starting or that the engine stalls, with the exhaust emission deteriorating.
It is to be assumed that the cause of the above problems is as follows: the heavy gasoline, which has a low vapor pressure, adheres to the intake pipe to cause a reduction in the amount of fuel actually supplied. Thus, at the time of or immediately after the engine start and when accelerating the engine, the actual air-fuel ratio is increased (i.e., becomes leaner), resulting in defective combustion.
In view of this, various devices have been proposed. For example, Japanese Patent Laid-Open No. 2000-8941 discloses a device which judges the fuel nature on the basis of the rotating speed fluctuation ratio immediately after the engine start, and Japanese Patent Laid-Open No. 2000-337207 discloses a device which judges the fuel nature on the basis of the condition from the engine start to the detection of the initial detonation.
In the fuel nature judging devices as described in the above-mentioned publications, however, when the engine is re-started after repeating one or two times a short-term engine start when the engine temperature is low (in which the engine is stopped after its start at an interval short enough not to cause a rise in the engine cooling water temperature), the fuel adhering to the intake pipe at the previous start is supplied when re-starting the engine. Thus, even when the fuel is a heavy gasoline, the actual air-fuel ratio is temporarily reduced (i.e., becomes rich) to cause a satisfactory combustion, which leads to an erroneous fuel nature judgment (i.e., the fuel is erroneously judged to be an ordinary gasoline).
When the engine temperature is high, the engine load is relatively low, so that even when the fuel is a heavy gasoline and the actual air-fuel ratio is lean, there is no reduction in engine rotating speed, and it is determined that a relatively satisfactory combustion is being effected. Thus, an erroneous judgment is made as to the fuel nature (i.e., the fuel is judged to be an ordinary gasoline).
Further, even in a case in which the cooling water temperature is not so high, when the cooling water temperature is higher than the ambient temperature, the period of time that has elapsed after the engine stop is relatively short. In such a case, the viscosity of the engine oil is low, and the engine load is relatively light, so that even when the fuel is a heavy gasoline and the actual air-fuel ratio is lean, it is determined that a relatively satisfactory combustion is being conducted. Thus, an erroneous fuel nature judgment is made (i.e., the fuel is judged to be an ordinary gasoline).
In the above-described conventional devices, the previous engine operating condition is not taken into consideration when performing fuel nature judgment at the time of and immediately after the engine start, so that when a heavy gasoline is being supplied, an erroneous fuel nature judgment can be made as a result of the satisfactory combustion with a temporarily rich air-fuel ratio due to the residual fuel or the relatively satisfactory combustion at the time of reduced engine load due to engine temperature.
This invention has been made with a view toward solving the above problem in the prior art. It is accordingly an object of this invention to provide a fuel nature judging device for an internal combustion engine which can prevent erroneous fuel nature judgment due to the previous engine operating condition and prevent erroneous fuel nature judgment when the engine load is light due to the current engine temperature.
In accordance with this invention, there is provided a fuel nature judging device for an internal combustion engine comprising a previous-engine-stop cooling water temperature storing means for storing a cooling water temperature at the time of previous engine start, a previous-engine-stop cooling water comparing means for comparing the cooling water temperature at the time of previous engine start with a first reference temperature, a current engine cooling water temperature comparing means for comparing a current cooling water temperature with a second reference water temperature, a cooling-water-temperature/intake-air-temperature deviation measuring means for measuring a deviation of the current cooling water temperature from a current intake air temperature, a deviation comparing means for comparing the deviation with a reference deviation, a fuel nature judgment permitting means which determines that fuel nature judgment requirements are satisfied and permits fuel nature judgment when the cooling water temperature at the previous engine stop is higher than the first reference temperature, when the cooling water temperature is lower than the second reference water temperature, and when the deviation is less than the reference deviation, a fuel nature judging means for judging fuel nature on the basis of a predetermined engine condition when the fuel nature judgment is permitted, and a fuel nature judgment result storing means for storing a judgment result of the fuel nature judging means.
Further, in this invention, when the cooling water temperature at the time of previous engine stop is not higher than the first reference temperature, when the cooling water temperature is not lower than the second reference water temperature, or when the deviation is not less than the reference deviation, the fuel nature judgment permitting means determines that the fuel nature judgment requirements are not satisfied and does not permit fuel nature judgment, and, in this case, the fuel nature judgment result storing means reads out the result of the previous fuel nature judgment, storing the judgment result again.
Further, in this invention, the cooling water temperature is measured by a cooling water temperature sensor mounted to the engine.
Further, in this invention, the intake air temperature is measured by an intake air temperature sensor mounted to an air cleaner connected to an intake pipe of the engine.